Cooperative communication enables wireless transmit/receive units (WTRUs) to assist each other in transmission of information to their desired destination. Such an approach enables mitigation of several issues facing modern wireless communication systems without the cost associated with extensive wired infrastructure. Using cooperation, it is also possible to exploit the spatial diversity associated with traditional multiple-input multiple-output (MIMO) techniques without requiring each node to have multiple antennas. Finally, regenerative relaying, a basic cooperative technique, may reduce the effects of path loss and shadowing on coverage and throughput.
A challenge in incorporating cooperation into modern wireless systems is the need to evolve the system architecture to enable cooperation. Effective cooperative techniques, especially in wireless systems, usually involve advanced algorithms at the lower layers of the communications stack, for example layer 1 (physical layer, or PHY) and layer 2/3 (medium access control (MAC), radio link control (RLC), or logical link control (LLC)—depending on the system). However, such algorithms require advanced techniques in receiver design, error-correction lisecode design, automatic repeat request (ARQ) and hybrid automatic repeat request (HARQ) processes and scheduling in multi-user systems.
There is therefore a need to consider the impact of cooperation on cellular systems, including system-architecture aspects. The downlink and uplink, separately and in each case, consider several cooperation schemes which result in different architectures. In each case, the impact on the system operation is considered, with emphasis on ARQ/HARQ and scheduling and solutions are proposed.
With the evolution of users' needs for various high quality and data rate services and applications, the capacities of wireless communication links are being exhausted. Single antenna systems are now being found to be unable to address these needs, and operators are now moving to multiple antennas systems. Despite their unprecedented achievable data rates, multiple antenna systems do not provide significant gain at far range or low signal-to-noise ratio (SNR) applications.
Relayed communication seems to address such an issue and is now the focus of many research activities. Unlike conventional point-to-point communication techniques, relaying introduces a third entity called a “relay” that assists in the communication between the source and the destination.
When assisting the source, the relay and the source agree to various protocols to deliver the intended message to its destination, for example hopping and diversity protocols. With hopping, the message is sent by the source, received by the relay and then retransmitted to the destination. With diversity protocols, the relay and the source simultaneously transmit to the destination using some diversity schemes.
The versatility introduced by the relay in terms of deployment and providing additional virtual antennas, are the key advantages of relaying systems. For example, multiple antennas are limited in size and cost, and thus are difficult to implement with more than four antennas. However, with relaying, the number of antennas in a link may be increased in a distributed manner, and thus can introduce higher gains in data rates. Also, by adjusting relay locations or by selecting the ones with the appropriate channel conditions, low SNR and far-range links receive a significant boost. Further, cell edge users are generally disfavored due to the high interference they experience. Relaying in this case can be used to increase and redistribute the throughput throughout the cell and enhance the disfavored links.
Notwithstanding these significant advantages of relaying and the extensive theoretical development in cooperative communication, little work has been performed towards introducing the benefits of cooperative communications to practical cellular systems. Some of the reasons for this are lack of efficient cooperation protocols with demonstrated benefits in real world scenarios and expensive implementations. Consequently, there is a need for cooperative communication protocols that are appropriate for cellular communications systems.
Relay communications has shown much promise recently in improving communications on weak communications links. By allowing the relay to transmit the full message to the destination in a multi-hop fashion, extremely remote communicating ends have been provided connectivity. However, multiple-hops result in communication delays that may be unacceptable in certain real-time applications.
A more improved structure for relayed communications is cooperative communications. Unlike multi-hopping, the source and the relay or multiple relays collaborate to provide diversity or multiplexing gains. As an example, the source and the relay could transmit in an Alamouti scheme. Relays are provided the option of decoding the message before helping or simply forwarding it after adapting its power to the channel. These techniques are called decode and forward (DF) and amplify and forward (AF), respectively.
The main disadvantage of these techniques is that delays are introduced by the relays when DF is assumed. One way to avoid this is to use a form of coding that allows the destination to collect data from the beginning of the communications while relays are receiving. By doing so, delays due to the DF protocols are reduced. The destination thus sees a continuous transmission throughout.
In another scheme, fountain codes, a special case of rateless codes optimally built for erasure channels, have been used for broadcast applications. However, there is a need for efficient use of rateless coding for practical relay systems.
Due to the propagation delay between the RS and BS, the frequency offset between the BS and RS local oscillators, as well as the processing delays in the RS, the timing of the RS transmissions to the WTRU may be different from the timing of the BS transmissions to the WTRU. During the cooperation phase, misalignment of the streams received by the WTRU from the BS and RS respectively may cause interference with each other. The inter-stream interference reduces the data rate that can be achieved by the WTRU, thus reducing the potential benefit from cooperation.
It would therefore be desirable to mitigate this problem by synchronizing the BS and the RS DL transmissions. Using synchronized BS and RS DL transmissions would help reduce the interference between the RS and the BS transmissions to the WTRU and enable the use of various diversity schemes (e.g. Alamouti or MIMO schemes) while avoiding complex WTRU receiver design.
Prior art solutions show that adjusting the timing of the uplink (UL) WTRU's transmission may be achieved through a timing adjust (TA) mechanism. While the TA concept is commonly used for the UL, so far it has not been used for the DL which is needed in the context of cooperative networks.
It would also be desirable to improve link performance through an intelligent use of relays. However, simple multi-hop relaying (i.e. one where the relay just forwards the same data that it receives) is not likely to result in significant gains. Instead, more sophisticated cooperative techniques may be employed. Among these are cooperative coding schemes, such as distributed beam-forming and distributed spatial multiplexing techniques. It would therefore be desirable to use a multi-user detector, more precisely a successive interference canceller (SIC), to optimize the performance of joint reception of transmissions from the source and relay. A minimum mean squared error successive interference canceller (MMSE-SIC) receiver is formally a candidate receiver for use in the Third Generation Partnership Project's (3GPP's) Long Term Evolution (LTE) technology for separating between spatial streams emanating from the same transmitter. Thus, it would be desirable to place the source and relay transmissions into separate transmission streams and use a SIC to receive these transmissions. In fact, at least for OFDM MIMO technologies, it may not even require additional receiver structures.
Specifically, the SIC receiver would be able to take advantage of apparent practicability and demonstrate that once such a receiver is introduced into a communication system, much of the advantage of cooperative diversity may be relegated to the MAC layer. Instead of collaborative transmission and coding, a well scheduled combination of direct transmission and simple multi-hop would be desirable to achieve the benefits of cooperative relays and, in some cases, even exceed what can be delivered by a well designed PHY-layer scheme.